The outer tube may have any shape; it may be smooth or it may be profiled in such a way as to obtain a lengthened leakage path, and it should be made of a dielectric material of sufficient mechanical strength (ceramics, glass, organic or inorganic fibres bonded in a synthetic resin, etc.).
Solid shanked electrical insulators made of ceramic material, of glass or of organic or inorganic fibres by a hardenable resin have excellent electrical properties, but it is difficult to manufacture such insulators having a large diameter, greater than about 200 to 250 mm for ceramic insulators or greater than an even smaller diameter (50 mm) for insulators of fiber-resin composite materials.
The use of tubular insulators may be considered; these can be manufactured with large diameters and/or great lengths and may be filled with a rigid foam filling of synthetic resin applied in situ for the purpose of providing electrical insulation and water-tight sealing. Nonetheless, there remains the problem of obtaining good adhesion between the rigid foam and the inner wall of the tube since the skin formed by the rigid foam on contact with the wall is brittle and runs the risk of not being able to withstand the strains likely to arise in use, which will reduce the electrical insulation properties. Further, it is practically impossible to make long insulators in this way which have substantially constant electrical insulation properties along their entire length. Finally, the application of the rigid foam in situ requires very tight working conditions (temperature control, mixing time for the constituants of the foam, etc.) and if these are not complied with there is no check on the quality of the foam, and faults such as air inclusions and unsticking from the inner wall of the tube cannot be avoided.
The present invention thus has the aim of providing a tubular insulator which may have a large diameter and/or great length and in which it is easy to check the quality of the filler of insulating material and to ensure permanent adhesion in service to the inner wall, thus maintaining its electrical insulating properties constant, but whose manufacture is nonetheless simple and does not require excessive precautions.